1. Field of the Invention
The present invention relates to an apparatus for controlling the speed of a motor which can prevent or suppress oscillation caused by torsion acting on a shaft between a load and the motor for driving the load and to a method for controlling the speed of a motor.
2. Description of the Related Art
An apparatus for controlling the speed of a motor for driving, as a load, a rolling machine for rolling for example steel, uses a speed control system as shown in FIG. 3.
As shown in FIG. 3, the speed control system comprises a speed control loop and a current control minor loop. The speed control loop comprises a speed detector 4 for detecting the rotational speed of a DC motor 3 to which a load 1 is coupled through a shaft 2, a first subtracter 5 for finding a difference between a command speed value and a detection speed value detected by the speed detector 4, and a speed controller 6, responsive to the speed difference from the first subtracter, for delivering a command current value based on a given control theory. The current control minor loop comprises a current detector 7 for detecting current in the motor 3, a second subtracter 8 for finding a difference between the command current value supplied from the speed controller 6 and a detection current value detected by the current detector 7, and a current controller 9, responsive to the current difference supplied from the second subtracter 8, for delivering a command voltage value to a control power source 10 on the basis of a given control theory. The speed control unit 6 and current control unit 9 are generally comprised of a proportional-plus-integral circuit.
In the case where the load 1 and motor of a control target are controlled in the speed control apparatus, a speed control system poses no problem if the shaft 2 coupling the motor to the load is sufficiently rigid. If, on the other hand, the shaft 2 exhibits sufficient elasticity, the oscillation caused by the torsion acting on the shaft is observed in the motor speed and load speed, for a reason as will be set out below, markedly lowering the speed controllability.
In the above motor speed control apparatus, the current control system incorporated as the minor loop is adapted to control current in an armature, that is, torque current proportional to the generation torque of the motor. It is therefore impossible to control acceleration torque directly related to the motor speed even if the torque acting on the shaft can be controlled.
As an example, Published Examined Japanese Patent Application 63-1839 discloses a motor speed control apparatus for suppressing oscillation caused by torsion acting on a shaft by which a motor is connected to a load.
FIG. 4 is a block diagram showing a motor speed control apparatus. The same reference numerals are employed in FIG. 4 to designate the same parts or elements corresponding to those shown in FIG. 3. The different parts or elements only will be explained below in more detail. As a system for controlling generation torque directly related to the motor speed, a shaft torque observer 30 is arranged in a feedback system relative to a current control system which receives the detection speed value and detection current value from the speed detector 4 and current detector 7. The shaft torque observer 30 implements an arithmetic operation based on the received detection speed value and detection current value to obtain an estimated shaft torque value and feeds the estimated value back to a second subtracter 8 in the current control system. In this case, a simulated value of the shaft torque is evaluated, by the arithmetic operation, from the detection speed value and detection current value as well as a mathematical model uniquely determined on the object to be controlled.
The motor speed control apparatus including the shaft torque observer 30 can be expected to suppress oscillation in the motor's shaft to some extent.
However, the shaft torque observer 30 sometimes fails to obtain a correct value of estimation because it involves a problem, such as an error of estimation or a rate of convergence to a true value. That is, the shaft torque observer 30 estimates the shaft torque through the mathematical operation based on a given theory, but, unlike the theory, some physical phenomenon, such as noise, emerges under the actual application circumstances. As such a phenomenon is not fully considered in the mathematical model, sometimes an estimated value of shaft torque entirely deviates from an observed value involved. In such a state, the speed control performance is degraded due to the presence of the shaft torque observer 30.
According to the "observer" theory, the shaft torque observer 30 requires a mathematical model for a to-be-controlled object for which observation is made, but it is not necessarily easy to obtain a correct mathematical model for the motor/load system. It is clear that the shaft torque, if being estimated based on an incorrect model, will deviate from the actual shaft torque.
Further, if the parameters of the object to be controlled, such as mechanical inertia and viscosity resistance, vary during the operation of the motor, even when the mathematical model is correct at one point of time or one point of operation, correct estimation cannot be expected throughout the whole operation period or over the whole operation range. If the mechanical inertia is involved for a system connecting rolls (rolls for a rolling machine) to a motor, for example, in a driving system for rolling steel or the like, it will prove insufficient for a reason set out below even if being incorporated into the mathematical model. During the rolling of steel, the mass of a steel sheet varies from the entering of it between the rolls until it leaves the rolled site. It is necessary at this time, to consider the mass involved. It is known that when coiling steel sheet by a coiling machine, the mass of the coiled sheet is gradually increased with an advance of the coiling operation and, hence, the mass of the load varies greatly between the time at which the coiling operation starts and its completion. It is not possible to precisely estimate shaft torque even if the mechanical inertia is unequivocally incorporated into the mathematical model.
Similarly, the parameters of the object to be observed are required to construct the mathematical model, but the parameters of the load vary each time one object to be observed is switched to another object to be observed, requiring a cumbersome task of adjusting the mathematical model each time.
In the conventional motor speed control apparatus, the shaft torque observer receives the detection speed value and detection current value from the motor speed and current detectors and estimates the shaft torque through the arithmetic operation based on the mathematical model and feeds a value of estimation back to the current control system to suppress oscillation caused by the torsion acting on the shaft by which the motor is coupled to the load. The shaft torque observer estimates the shaft torque from the measured speed and current in the motor. These variables are produced only after the torque changes in the shaft have produced speed changes in the motor. Therefore the observer estimates of torque is delayed in time from the actual torque. It is, therefore, not possible to obtain a correct value of estimation. The mathematical model, in particular, is unequivocally determined, depending upon the object to be controlled. Since, in practice, a physical phenomenon different from the mathematical model may occur during the operation of the motor and load, the estimated value of shaft torque is sometimes entirely deviated from an observation value, posing a reliability problem.